1. Field of the Invention
The invention relates to a gas chromatograph comprising a chromatographic separating device and a downstream absorption spectrometer, containing a wavelength-tunable light source, for quantitative determination of separated components of a sample of a gas mixture conveyed through the separating device via a carrier gas.
2. Description of the Related Art
The invention furthermore relates to a method for analyzing a gas mixture via gas chromatography, where a sample of the gas mixture is passed through a chromatographic separating device by means of a carrier gas and separated components of the gas mixture are quantitatively determined in a downstream absorption spectrometer having a wavelength-tunable light source.
A gas chromatograph and method are known from U.S. Pat. No. 7,511,802 B2. In the known gas chromatograph and method, after leaving the separating device each individual component is introduced through a switch valve into an analysis cell of the absorption spectrometer. The light of the wavelength-tunable light source is passed through the analysis cell onto a photodetector, the wavelength of the light source being tuned to an absorption line of the component to be determined. The light absorption of the component is dependent on its concentration in the analysis cell, so that the output signal generated by the photodetector is a measure of this concentration. Since the light absorption is very weak, the light beam is conveyed via reflection several times through the analysis cell before it strikes the photodetector. After each determination of a component, the analysis cell is flushed with the carrier gas via the switch valve. Insufficiently separated components may be passed together into the analysis cell and determined there, if they have non-overlapping absorption lines to which the wavelength of the light source can be tuned. In the case of overlapping absorption lines, there is the possibility of separating the components by gas scrubbing and then delivering them individually to the analysis cell. Nitrogen, argon or helium, which are not infrared-active and therefore do not interfere with the absorption spectra of the components to be determined, may be envisioned as the carrier gas.
WO 2008/061949 A1 and WO 2011/026924 A1 respectively disclose an absorption spectrometer in which the analysis cell is configured as a waveguide, in which the gas to be analyzed is contained. The light of the light source is input into the waveguide at one end thereof and output at the other end onto the photodetector. The light is guided by reflection in the optionally internally mirrored waveguide, so that the waveguide may be curved and therefore formed as a hollow fiber. In order to reduce artefacts, such as interference, the waveguide may be set in vibration. The advantage of this known absorption spectrometer resides in the small measurement volume and the long optical measurement path.
In connection with the known determination via absorption spectrometry of components previously separated via gas chromatography, as already mentioned above, the following problems arise:                With wavelength-tunable light sources commercially available at present, generally lasers or laser diodes, only a very restricted number of gas components can be determined; for example, noble gases or nitrogen in principle and higher hydrocarbons in practice cannot be measured.        The tuning of the light source to the absorption lines of the various components is elaborate and time-consuming.        The carrier gases used are expensive.        